Thermoelectric transport coefficients of n-doped CaTiO3, SrTiO3 and BaTiO3: A theoretical study

Boltzmann transport equations and density functional theory calculations were employed to calculate the thermoelectric transport coefficients of CaTiO₃, SrTiO₃ and BaTiO₃. It was found that BaTiO₃ has the largest Seebeck coefficient and power factor. Then the transport coefficients were analyzed using the ‘Tight Binding Model’. The band narrowing, caused by the increasing lattice constants from CaTiO₃ to BaTiO₃, was the main reason for the increasing Seebeck coefficients and the decreasing electrical conductivity. The calculated electrical conductivity and electronic thermal conductivity were in line with the Wiedemann-Franz law and the Lorenz factor was determined to be 2.45 for these oxides as degenerate semiconductors. Our theoretical results are helpful for seeking high performance thermoelectric oxides.     

掺Ag纳米颗粒的BaTiO3复合薄膜的非线性光学特性

通过准分子激光(XeCl, 308 nm, 20 ns)在MgO(100)单晶衬底上制备了不同掺杂浓度的Ag:BaTiO₃纳米复合薄膜,通过X射线衍射和X射线光电子能谱对薄膜的结构和组分进行了表征.在430 nm和470 nm附近观测到了不同浓度Ag纳米颗粒引起的等离子体吸收峰,通过z扫描技术对复合薄膜的三阶非线性光学特性进行了测量,并对其光学非线性的增强机制进行了讨论. 关键词： 金属纳米复合薄膜 激光沉积 非线性光学     

Doping effects of Nb5+ on red long afterglow phosphor CaTiO3:Pr3+

A series of Nb5+ codoped red long afterglow phosphors CaTi1 xNbxO3:Pr03.+002 (0 ≤ x ≤ 0.05) is prepared by a solid state reaction method. Their photoluminescence, phosphorescence and thermoluminescence are investigated. The results indicate that codoping Nb5+ can improve the photoluminescence and phosphorescence property of CaTiO3:Pr3+ significantly. When 3-mol% Nb5+ is codoped, the emission intensity of CaTiO3:Pr3+ is enhanced twice, while the afterglow time is extended from 10 min to about 40 min. Thermoluminescence results reveal that the trapping level of CaTiO3:Pr3+ is reduced from 0.82 eV to 0.62 eV by codoping Nb5+. The effect of Nb5+ doping on enhancing the photoluminescence intensity and afterglow time of CaTiO3:Pr3+ is discussed.     

Quantitative analysis of UV excitation bands for red emissions in Pr-doped CaTiO3, SrTiO3 and BaTiO3 phosphors by peak fitting

A quantitative spectral analysis of the ultraviolet (UV) broad excitation bands, which are located in the range 300-400 nm, for red emissions at around 610 nm in Pr-doped CaTiO₃, SrTiO₃:Al and BaTiO₃:Mg phosphors has been carried out using a peak fitting technique. The obtained results demonstrate that the UV broad band of CaTiO₃:Pr consists of four primary excitation bands centered around 330, 335, 365 and 380 nm and those of both SrTiO₃:Al and BaTiO₃:Mg consist of three primary bands centered around 310, 345 and 370 nm. Based on the behavior patterns and the values of the respective primary excitation bands’ parameters, i.e. center gravity (λ_top), maximum height (I_max) and full-width at half-maximum (FWHM), the UV-to-red relaxation processes in these titanate phosphors can be explained to be essentially the same, except for the existence of an additional relaxation pathway via electron-trap states in CaTiO₃:Pr, which gives a characteristic shape of its UV excitation spectrum in the wavelength range of >360 nm.     

Positron annihilation in BaTiO3

The angular correlation of 2 γ-annihilation radiation has been measured in barium titanate single crystals. The effect on the angular correlation of the ferroelectricity in BaTiO₃ was not detected.     

Effect of annealing atmosphere on magnetism for Fe-doped BaTiO3 ceramic

Ba(Ti_0.3Fe_0.7)O₃ ceramic was prepared by solid-state reaction and post-annealed in vacuum and oxygen, respectively. The as-prepared and annealed samples are all single-phase, crystallizing in a 6H-BaTiO₃-type hexagonal perovskite structure. Room-temperature ferromagnetism is exhibited in all ceramics. For the as-prepared sample, the super-exchange interactions of Fe³⁺ in different occupational sites (pentahedral and octahedral sites) are expected to produce the ferromagnetism observed. After annealing in vacuum, the magnetization is reduced while the exchange mechanism remains unchanged. On the contrary, O₂ annealing can effectively enhance the magnetization due to the presence of Fe⁴⁺, an unusual valence for iron. The simultaneous presence of Fe³⁺ and Fe⁴⁺ allows new exchange mechanism responsible for the ferromagnetic interaction. The exchange couplings of Fe ions with mixed valences (Fe³⁺ and Fe⁴⁺) determine the magnetic behavior.     

X-ray critical scattering in BaTiO3

Critical scattering of X-rays in a BaTiO₃ monocrystal was measured around the [005] reciprocal lattice point. The constant intensity curves in reciprocal lattice space are exactly analogous with those determined by Yamada et al. around the [200] point.     

Rotation of ferromagnetic clusters induced magnetoresistance in the junction composed of La0.9Ca0.1MnO3 +δ and 1 wt.% Nb-doped SrTiO3

A junction composed of ultrathin La0.9Ca0.1MnO3＋δ （LCMO） film and 1 wt.% Nb-doped SrTiO3 was fabricated and its magnetoresistance （MR） was studied and compared with LCMO film. It was found that the resistance of the junction has a similar dependence on magnetic field as that of the LCMO film： the curvature of R-H curves is upward above Curie temperature （Tc） and downward below TC. These behaviours strongly suggest that the rotation of ferromagnetic clusters in manganite also causes MR in the corresponding junction. This MR can be qualitatively understood by the change of the width of the barrier induced by the rotation of ferromagnetic clusters. These results suggest a possibility to obtain junctions with large low-field MR.     

Dielectric response in ferroelectric BaTiO3

The far-infrared optical and dielectric properties of ferroelectric perovskite titanate powder BaTiO₃ are reported. The terahertz time-domain spectroscopy (THz-TDS) measurement reveals that the low frequency dielectric response of BaTiO₃ is closely related to the lowest pair of transverse optical (TO) and longitudinal optical (LO) modes near at 180 cm⁻¹, which is verified by Raman spectroscopy. This result provides a better understanding of the relation of low-frequency dielectric function with the optical phonon mode for ferroelectric materials. Combining terahertz TDS with Raman spectra, the overall low frequency optical phonon response of BaTiO₃ is presented in an extended spectral range from 6.7 to 1200 cm⁻¹.     

Bulk photovoltaic effect in BaTiO3

In melt-grown BaTiO₃ single crystals steady-state photocurrents proportional to the light intensity have been observed parallel to the crystallographic c-axis. The open-circuit photovoltage exceeds the value of the band gap. Light polarized parallel and perpendicular to the c-axis respectively produces photovoltages with opposite sign. This photovoltaic effect is restricted to the ferroelectric phase.     

金属纳米团簇复合薄膜Au/BaTiO3与Fe/BaTiO3的PLD制备及其光吸收特征

用脉冲激光沉积技术制备了掺杂纳米金属颗粒Au或Fe的BaTiO₃复合薄膜.用透射电子显微镜和x射线光电子能谱表征了金属颗粒的形态和化学态.330—800nm范围的吸收谱研究表明，掺Au颗粒的BaTiO₃薄膜在580nm附近有一个明显的共振吸收峰，而掺Fe颗粒的BaTiO₃薄膜没有这样的吸收峰.用Mie散射理论对结果进行了分析. 关键词： 复合薄膜 金属颗粒 脉冲激光沉积 吸收谱     

Magnetic and electrical properties of bulk BaTiO3+MgFe2O4 composite

BaTiO₃+MgFe₂O₄ material system was synthesized by hybrid chemical process using chlorides and nitrates of barium, titanium, iron, and magnesium. Magnetic properties of the composite samples measured as a function of annealing conditions indicated soft magnetic behavior. Saturation specific magnetization from 8 21 emu/g was observed for samples annealed at temperature between 950 and 1150 °C. Variation of specific saturation magnetization with respect to annealing temperature was related with the distribution of Fe cations in the tetrahedral and octahedral sites of MgFe₂O₄. Electrical properties of the samples annealed at different temperatures were measured to analyze the coexistence of ferroelectric phase. Dielectric constant varying from 15 to 200 with respect to frequency was observed for samples annealed from 950 to 1150 °C.     

Nanocrystalline CaTiO3 prepared by soft-chemical route

Nanocrystalline CaTiO₃ has been synthesized by using soft-chemical route. X-ray diffraction study indicates that as-prepared powders were amorphous in nature and formation of CaTiO₃ phase starts at about 773 K. Phase formation saturates at around 1023 K and it remains orthorhombic up to 1400 K. Microstructure analysis by the Rietveld method revealed that the CaTiO₃ crystal size varies from 22.9 to 49.3 nm, while the annealing temperature varies from 773 to 1023 K. The variation of DC resistivity with temperature in the range 400–560 K shows a polaronic behavior in this system.     

BaTiO3和Ce:BaTiO3单晶的高温喇曼散射研究

从室温至180℃测量了BaTiO₃和Ce:BaTiO₃单晶的偏振喇曼谱,在X(ZZ)Y几何配置下发现了三条频率分别为986,1204和1480cm^-1的新谱线.根据喇曼散射截面的温度依赖关系,确认这些新谱线对应二阶喇曼散射,而A₁(TO)谱中位于275和514cm^-1处的非对称宽峰则属于一阶喇曼散射.在此基础上,对立方相BaTiO₃的喇曼谱和结构相变机制进行了讨论.通过比较B     

Holographic storage properties of BaTiO3

BaTiO₃ single crystals have been grown for the storage of volume phase holograms via photorefractive effects. The storage properties are derived from measurements with incoherent light and the results are examined with the help of holographic experiments.     

YBa2Cu3O6+δ/BaTiO3系复合功能陶瓷特性研究

报道了超导陶瓷ＹＢａ₂Ｃｕ₃Ｏ_6+δ与铁电陶瓷ＢａＴｉＯ₃进行复合的结果。研究了该复合功能陶瓷的物相、（超）导电性和低温电阻温度特性。结果表明，采用合理的合成工艺，可得到呈现混和分布的两相复合功能陶瓷材料；该复合材料的电导特征符合三维渗流导电行为，发现在较高ＹＢａ₂Ｃｕ₃Ｏ_6+δ含量时，样品呈超导电性，并对此作了初步讨论。 关键词：     

Surface segregation in Nb-doped BaTiO3 films

We have used in situ photoemission spectroscopy to investigate Niobium doping in polycristalline BaTiO₃. The valence band maximum position progressively shifts from 2.5 eV for undoped to 2.84 eV for Nb-doped films. Ceramics and single crystal have been investigated for comparison with thin films. Nb-doped BaTiO₃ ceramics and Nb-doped SrTiO₃ single crystal show higher Fermi level position indicating that our doped films are less conducting regarding their bulk parents. This was confirmed by impedance spectroscopy under variable temperature. Large amount of niobium is clearly observable at surface but the amount of dopant is drastically reduced below the near-surface region, as evidenced by depth profile. Therefore, we provide evidence of surface segregation which would explain the contrasted resistivity values reported in literature for such donor-doped films.     

Y3Al5O12:Ce3+ fluorescent ceramic for optical data storage

A kind of optical data storage medium based on electron-trapping materials,Y₃Al₅O₁₂:Ce³⁺fluorescent ceramic,was developed by vacuum sintering technology.The medium shows sufficiently deep traps[1.67 and 0.77 eV).The properties of trap levels were researched by thermoluminescence curves,and the optical storage mechanism based on Ce³⁺ion doping was proposed.More importantly,the data can be written-in by 254 nm UV light,and readout by heating[3...     

钙钛矿CaTiO3的超高压结构研究

利用同步辐射x射线衍射和 DAC 高压技术在室温下测量了钙钛矿CaTiO₃在压力0—44.53 GPa下的结构变化.结果表明，随着压力的增加CaTiO₃的三个晶轴都受到不同程度的压缩，a,b的压缩率相近且相对比较大，c的压缩率最小，但没有证据表明有相变的发生.在压力范围内CaTiO₃的P-V关系用Murnaghan状态方程表示，设定K′0=4，得到V0=0.2245(6)nm3和K0=222(9) GPa.应用赝立方角γpc与压力的关系，初 关键词： 3')" href="#">CaTiO₃ 结构 超高压 状态方程     

Spin-spin relaxation in slightly ferroelectric KH2AsO4 and BaTiO3:Mn2+

An approximate theory of dipolar relaxation by the soft modes of magnetic vacancies and dislocations gives $\text{2}\text{3}$ and 1 for the power-law exponents of T₂, while proton T₁ and EPR linewidth measurements yield 0.67 ± 0.04 and 1.0 ± 0.1.